This invention relates generally to printed wiring boards and, more particularly, to a technique and the resultant product for forming printed wiring boards wherein the technique includes laminating at least two layers together to form the printed wiring board.
High frequency applications in the field of printed wiring boards, i.e. one GHz and above, are driving the need for smooth copper features on the surfaces of the signal lines. This need is due to the so-called skin effect where, as frequency increases, the path of the electrical signal tends toward the outer surface of the conductor. Hence, roughness on the surface of the copper in high frequency applications will result in higher surface resistivity and longer effective line length, both of which contribute to higher conductive losses for the signal. Also, signal integrity can be affected by the roughness in the ground or voltage planes that are referenced by a signal line in a composite board structure.
However, conventional printed wiring board processes, such as lamination, depend on roughened copper surfaces in order to provide adequate adhesion of the copper to dielectric laminate in the composite laminated structure. Typically, the exposed surfaces of internal wiring planes and voltage planes of a layer prior to lamination are initially smooth, and then are roughened to promote adhesion. Techniques for roughening the copper include the oxide and oxide replacement processes, as well as the application of brass, or zinc and/or nickel on the copper surface. Conventionally, the roughening treatment is applied to all the exposed copper surfaces prior to, as well as after, personalization of the copper plane. Thus, the two competing problems require different surface roughnesses for optimum benefit; i.e., a very smooth surface of the conductive material is desired for a most efficient signal propagation, while a roughened surface is desired for optimum adhesion of copper to the dielectric material.
It has been found that roughening of the conductors on the printed wiring board structure is critical only in certain regions and not required for the entire length of each of the circuit traces or signal lines. In fact, the mechanical and chemical exposures are greatest where a signal or power plane intersect a plated through hole. Therefore, the need is greater for good copper to laminate adhesion at this intersection than in the open, non-drilled areas of the board. Thus, according to the present invention, a printed wiring board is formed from two or more layers, one of which has circuit lines formed thereon, and wherein the surfaces of the circuit lines or traces are selectively roughened only in those areas that require very good copper to laminate adhesion, whereas the remainder of the surface of the circuit lines or traces are maintained in essentially a smooth condition. This provides a good solution to the conflicting needs for good adhesion and good signal propagation qualities since there is only a limited or relatively small area that requires very good adhesion, and these areas are generally so small that they do not materially affect the propagation of the signals on the signal lines. Thus, those critical areas for propagation of the signal on signal lines or traces can have the circuit lines or traces smooth to maximize the signal propagation effect, while those limited areas where the signal propagation is not critical can be roughened so as to improve the adhesion of one layer to another. Therefore, in the resulting board, adequate adhesion can be obtained while still providing a significantly better signal propagation than is possible with the roughened conductor surface.
It has also been found that on the voltage planes (including power and ground planes) smoothing the surface of the voltage plane in those regions opposite the smooth surface regions of the signal planes improves the performance of the signal propagation. Thus, these limited areas of the voltage planes can be maintained smooth while the other areas of the surface of the voltage planes can be roughened, which provides the necessary adhesion of the voltage plane to the adjoining layer of dielectric material between the voltage plane and the signal plane.
In the case of both signal and voltage planes, the application of selective roughening is not necessarily dictated solely by the location of signal lines and plated through holes, but can be customized for a specific board design by balancing the electrical signal performance characteristics and mechanical requirements of that board. For example, one design may require smooth conductors on every signal line and the respective area of the reference planes, another design may prescribe smooth conductors on the signal lines only and not the reference planes, while still another may have only a few select number of signal lines requiring smooth conductors for optimum electrical performance, allowing all other conductors roughened for maximum mechanical adhesion.